In UTRA TDD, new User Equipment (UE) firstly synchronises with the network and then transmits a random access burst using the Physical Random Access Channel (PRACH). The PRACH burst is actually received at the Node B (UMTS base station) a time t (directly proportional to the distance between the UE and Node B) later. A guard period is thus required in order to avoid interference at the Node B between the PRACH burst and another burst in the subsequent timeslot. The size of the guard period effectively limits the cell radius under normal operation.
However, the network receiver (Node B) attempts to detect the random access burst by identifying which of a possible finite set of training sequences (midambles) was transmitted by the UE, and typically this operation is performed by correlating the appropriate received samples with a local replica of each of the allowable training sequences. From each of the correlator outputs the presence of PRACH burst(s) with that particular training sequence can be ascertained. Due to the construction of the training sequences from a single periodic base code, a high probability of false PRACH detection is experienced if the cell radius (allowable distance between UE and Node B) is allowed to be too large.
The maximum cell size is thus significantly limited below that allowed by the duration of the guard period, and is consequently not ideally suited to coverage of wide areas.
Although within UTRAN there currently exists an option to only use the odd-numbered midambles derived from a single periodic code, the resultant cell size is still not ideally suited to wide area coverage and does not take full advantage of the PRACH burst guard period duration.
An alternative to the above approach for channel estimation could be to employ a zero-forcing channel estimate. However, in an asynchronous CDMA system with a potentially large cell radius, a zero-forcing channel estimate would result in a degradation in signal-to-noise ratio too high for the reliable detection and demodulation of the PRACH bursts.
A need therefore exists for PRACH detection in a wireless communication system wherein the abovementioned disadvantage(s) may be alleviated.